1. Field of the Invention
The present invention relates to a magnetron, and more particularly to a magnetron including vanes of a positive polar portion which generates microwaves and have slanted and circular arc shape, thereby capable of reducing materials and decreasing its size.
2. Description of the Prior Art
Generally, a magnetron of a microwave oven is a device for generating microwaves by converting electrical potential energy into a high frequency energy. The magnetron is utilized as a heat source for inciting a frictional heat between molecules during the thawing or cooking of food.
FIG. 1 shows a vertical section structure of a magnetron generally used for a microwave oven. A negative polar portion includes a filament 10 disposed at the center line. Filament 10 is supported by a center lead 14 and a side lead 18. Center lead 14 is connected to one end of filament 10 via an upper shield 12 and a side lead 18 is connected to the other end of filament 10 via a lower shield 16. A positive polar portion includes a positive polar cylinder 20 and a plurality of vanes 22. Vanes 22 protrude from the inner wall of positive polar cylinder 20 apart from filament 10. Vanes 22 comprise two groups of alternating vanes, one group interconnected by an outer strap ring 24, and the other group interconnected by an inner strap ring 26. Annular permanent magnets 28 and 30 are installed to the upper and lower sides of positive cylinder 20. Magnetic fluxes flow from upper permanent magnet 28 to lower permanent magnet 30 via an activating space 32 secured between filament 10 and vanes 22 so that a uniform magnetic field is formed in the cylindrical axial direction. A magnetic circuit includes magnet members such as upper permanent magnet 28, an upper yoke 34, a lower yoke 36, lower permanent magnet 30, etc. Electrons emitted from filament 10 of a negative potential flow toward the radially inward facing surfaces of positive vanes 22 of a ground potential. Electrons circulate in activating space 32 due to the Lorentz force created by the electric field making a right angle with a magnetic field. By doing so, the electric field of high frequency reaches the radically inward facing end of positive vanes 22 to generate high frequency oscillation in a cavity resonator in the positive inner circumference. A high frequency voltage generated as described above radiates the microwaves produced by a high frequency electric field via an antenna lead 38.
As described above, the high frequency oscillation is affected by the resonant frequency of the cavity resonator. The resonant frequency is influenced by the size of a cavity formed by a pair of adjacent vanes 22 and inner wall of positive cylinder 20.
FIG. 2 illustrates a plan structure of positive polar cylinder 20 and vanes 22, and FIG. 3 illustrates a vertical section structure of vanes 22 along line 2-2' of FIG. 2. Vanes 22 are radially extending from the inner wall of positive polar cylinder 20 toward the center. Therefore, the cavity resonator is formed by a cavity 39 defined by each pair of vanes and inner wall of positive polar cylinder 20. The inductance of the cavity resonator is affected by a length L of the pair of vanes extending from a root portion 21 to an end portion 23, and a capacitance is affected by plane areas of adjacent vanes facing with each other. The longer the vane, the higher the inductance; and, the larger the area of the vane, the higher the capacitance. The resonant frequency is inversely proportional to the square root of the multiplication of inductance and capacitance. For this reason, as the size of the vane is decreased, the resonant frequency becomes increased.
The magnetron is set at a regular resonant frequency, in which, in order to create the set resonant frequency, the positive cylinder and vanes are designed to have predetermined dimensions. Furthermore, the positive cylinder and vanes are fabricated by using a highly-purified (i.e., oxygen-free, high conductivity) process for providing a tolerance to high temperature oscillation and vibration. The OFHC process is expensive to raise the cost of the magnetron.